1. Field of the Invention
This invention relates to a vacuum pump, and more particularly, to a vacuum pump for the low pressure pumping of fluids which may be used with microsensors and a mass-spectrograph in particular.
2. Description of the Prior Art
Various devices are currently available for determining the quantity and type of molecules present in a gas sample. One such device is the mass-spectrograph.
Mass-spectrographs determine the quantity and type of molecules present in a gas sample by measuring their masses. This is accomplished by ionizing a small sample and then using electric and/or magnetic fields to find a charge-to-mass ratio of the ion. Current mass-spectrographs are bulky, bench-top sized instruments. These mass-spectrographs are heavy (100 pounds) and expensive. Their big advantage is that they can be used in any environment.
Another device used to determine the quantity and type of molecules present in a gas sample is a chemical sensor. These can be purchased at a low cost, but these sensors must be calibrated to work in a specific environment and are sensitive to a limited number of chemicals. Therefore, multiple sensors are needed in complex environments.
A need existed for a low-cost gas detection sensor that will work in any environment. U.S. Pat. No. 5,386,115, hereby incorporated by reference, discloses a solid state mass-spectrograph which can be implemented on a semiconductor substrate.
FIG. 1 illustrates a functional diagram of such a mass-spectrograph 1. This mass-spectrograph 1 is capable of simultaneously detecting a plurality of constituents in a sample gas. This sample gas enters the spectrograph 1 through dust filter 3 which keeps particulate from clogging the gas sampling path. This sample gas then moves through a sample orifice 5 to a gas ionizer 7 where the gas is ionized by electron bombardment, energetic particles from nuclear decays, or in an electrical discharge plasma. Ion optics 9 accelerate and focus the ions through a mass filter 11. The mass filter 11 applies a strong electromagnetic field to the ion beam.
Mass filters which utilize primarily magnetic fields appear to be best suited for the miniature mass-spectrograph since the required magnetic field of about 1 Tesla (10,000 gauss) is easily achieved in a compact, permanent magnet design. Ions of the sample gas that are accelerated to the same energy will describe circular paths when exposed in the mass-filter 11 to a homogenous magnetic field perpendicular to the ion's direction of travel. The radius of the arc of the path is dependent upon the ion's mass-to-charge ratio.
The mass-filter 11 is preferably a Wien filter in which crossed electrostatic and magnetic fields produce a constant velocity-filtered ion beam 13 in which the ions are disbursed according to their mass/charge ratio in a dispersion plane which is in the plane of FIG. 1.
A vacuum pump 15 creates a vacuum in the mass-filter 11 to provide a collision-free environment for the ions. This vacuum is needed in order to prevent error in the ion's trajectories due to these collisions.
The mass-filtered ion beam is collected in a ion detector 17. Preferably, the ion detector 17 is a linear array of detector elements which makes possible the simultaneous detection of a plurality of the constituents of the sample gas. A microprocessor 19 analyses the detector output to determine the chemical makeup of the sampled gas using well-known algorithms which relate the velocity of the ions and their mass.
The results of the analysis generated by the microprocessor 19 are provided to an output device 21 which can comprise an alarm, a local display, a transmitter and/or data storage. The display can take the form shown at 21 in FIG. 1 in which the constituents of the sample gas are identified by the lines measured in atomic mass units (AMU).
Preferably, a mass-spectrograph 1 is implemented in a semiconductor chip 23 as illustrated in FIG. 2. In the preferred spectrograph 1, chip 23 is about 20 mm long, 10 mm wide and 0.8 mm thick.
Chip 23 comprises a substrate of semiconductor material formed in two halves 25a and 25b which are joined along longitudinally extending parting surfaces 27a and 27b. The two substrate halves 25a and 25b form at their parting surfaces 27a and 27b an elongated cavity 29. This cavity 29 has an inlet section 31, a gas ionizing section 33, a mass filter section 35, and a detector section 37. A number of partitions 39 formed in the substrate extend across the cavity 29 forming chambers 41. These chambers 41 are interconnected by aligned apertures 43 in the partitions 39 in the half 25a which define the path of the gas through the cavity 29.
A vacuum pump 15 may be connected to each of the chambers 41 through lateral passages 45 formed in the confronting surfaces 27a and 27b. This arrangement provides differential pumping of the chambers 41 and makes it possible to achieve the pressures and pump displacement volume or pumping speed required in the mass filter 11 and detector sections with a miniature vacuum pump 15.
In order to evacuate cavity 29 and draw a sample of gas into the spectrograph 1, the vacuum pump 15 must be capable of operation at very low pressures. Moreover, because of size constraints, vacuum pump 15 is preferably micro-miniature in size.
Although a number of prior art micro-pumps have been described, these pumps have generally focused on the pumping of liquids. In addition, micro-pumps have been used to pump gases near or higher than atmospheric pressure. Moreover, such micro-pumps are fabricated by bulk micro-machining techniques wherein several silicon or glass wafers are bonded together. This is a cumbersome procedure which is less than fully compatible with integrated circuit applications.
Other conventional micro-pumps utilize moving parts such as diaphragms and rotating or sliding shaft feedthroughs. Such micro-pumps are subject to wear and replacement. Conventional piston pumps may introduce undesired pulsations into the gas pressure and flow and may be relatively noisy. Furthermore, some conventional pumps require oil for lubrication and the oil may react with the gases being pumped.
Conventional dynamic vacuum pumps have been constructed which utilize thermal transpiration to obtain pressure rises. Thermal transpiration is discussed in Knudsen, M., Eine Revision der Gleichgewichtsbedingung der Gase, Annalen der Physik, 31, 205-229 (1910), which is incorporated herein by reference.
Thermal transpiration may be described in the context of two large volumes V.sub.c, V.sub.H of length L which are interconnected by a small tube having a radius R. Under equilibrium conditions, and for a continuum flow regime (where the mean free path length of the molecules is much smaller than the length of the large volumes; i.e. .lambda.&lt;&lt;L) then the pressure in both volumes will be the same and the density related to the temperature ratio, namely
P.sub.C =P.sub.H and .rho..sub.H /.rho..sub.C =T.sub.C /T.sub.H PA1 P.sub.H /P.sub.C =(T.sub.H /T.sub.C).sup.1/2 and P.sub.H /P.sub.C =(T.sub.C /T.sub.H).sup.1/2 PA1 P.sub.high /P.sub.low =(T.sub.H /T.sub.C).sup.N/2
However, if the radius R of the small tube is sized such that the gas inside it is in a free molecular flow regime (i.e. R&lt;&lt;.lambda.) and the two volumes are still in a continuum regime, then the pressures in the two volumes are related by
For example, for a temperature difference of 600K and 300K, the hot side pressure is 2.sup.1/2 =1.414 greater than the cold side pressure.
Further, multiple stages may be strung together to produce a significant pressure rise. Specifically, for N stages
This relationship applies even when the tube length is shortened to such a degree that only a thin aperture connects the two volumes provided that the gas inside the tube is in a free molecular flow regime and the two volumes are still in a continuum regime.
Conventional pumps which utilize thermal transpiration are macroscopic bench-top or larger units which have been laboriously fashioned.